Pentadienyl type lithium and potassium species: the regioselectivity of their reactions with electrophiles

Seven structurally distinct pentadienyl type lithium and potassium compds. were screened against a variety of electrophiles in order to assess the regioselectivity of the trapping reactions. Organoborates and analogs thereof (fluorodimethoxyborane) proved to be perfectly regio-reliable attacking only unsubstituted terminal positions and thus providing, after oxidn., exclusively primary allylic alcs. 2,4-Pentadienyllithiums or -potassiums, that carry a Me group at the 1- or 3-position, exhibit the same extreme regioselectivity towards halotrialkylsilanes or carbon dioxide. Although the unsubstituted parent compds. combine with such electrophiles still preferentially at the terminal position, considerable proportions of branched products are concomitantly formed as well (1/3-attack ratios ranging from 2:1 to > 20:1). Hydroxyalkylating and alkylating reagents such as formaldehyde, oxirane or Bu iodide invariably afford regioisomeric mixts. generally varying in compn. between 3:1 and 1:3. The condensation reaction with halotrialkylsilanes appears to follow a concerted (SN2-like) rather than an addn./elimination (ate complex mediated) mechanism. [on SciFinder (R)]